1. Field of the Invention
The present invention relates to an alcohol production method by reducing an ester or a lactone with hydrogen.
2. Brief Description of the Related Art
Methods for obtaining alcohols by reducing esters and lactones are important in chemical synthesis. For obtaining such reactions, the following methods have been proposed: a method in which a stoichiometric amount or more of a metal hydride compound such as silyl hydride, sodium borohydride and lithium aluminium hydride is used; and a method in which catalytic hydrogenation reduction is conducted using molecular hydrogen. The former method has problems including: a great amount of waste generated from a reducing agent; and a safety concern raised by the use of a highly-reactive reducing agent. For this reason, recently, the latter method which is an environmentally friendly technology has been more actively developed and examined, regardless of heterogeneous and homogeneous reductions.
For example, Japanese Unexamined Patent Application Publication No. Sho 51-8203 and Org. React., 1954, 8, 1 propose examples of the heterogeneous hydrogenation reduction. However, such proposals have a problem that a high temperature condition or a high pressure condition, or both of the conditions are needed for the reduction, and other problems. Meanwhile, Adv. Synth. Cat., 2001, 343, 802 describes a production method under such a condition that racemization of an optically active ester may not be involved. However, the production method has a problem that, when the ester serving as the substrate has an aromatic ring, side-reactions frequently occur to produce, for example, alcohols with the aromatic ring reduced, thus exhibiting a low selectivity. Moreover, the production method has a disadvantage in terms of cost that a large amount of very expensive catalysts have to be used. Accordingly, the method is difficult to industrially put into practical use.
As for the homogeneous reduction, the utilization of a ruthenium complex including a phosphine ligand has been proposed in many documents.
For example, J. Am. Chem. Soc. 1981, 103, 7536, Chem. Commun. 1998, 1367, and J. Mol. Catal. A: Chem., 2003, 206, 185 disclose methods in which monophosphine, diphosphine, triphosphine and tetraphosphine ligands, and the like are used. Particularly when a tridentate triphosphine ligand is used, a relatively high hydrogenation activity is demonstrated. However, when an ester is not activated for a reduction, a fluorinated compound having a large environmental load has to be used as a solvent. This kind of problem makes the methods difficult to industrially put into practical use. Angew. Chem. Int. Ed. 2006, 45, 1113 and Organomet. 2007, 26, 16 disclose hydrogenation reduction of esters with a ruthenium complex including a tridentate diaminophosphine or aminodiphosphine ligand. In the method of Angew. Chem. Int. Ed. 2006, 45, 1113, carbon tetrachloride used when a ligand and a complex are prepared is environmentally harmful, and its production is banned in most of the world. Moreover, the reaction needs to be conducted at a low temperature. Accordingly, the method has a lot of industrial disadvantages. The method disclosed in Organomet. 2007, 26, 16 has difficulties that: a complex is prepared while being irradiated with microwaves; the hydrogenation of esters needs to be conducted at a high temperature of 140 to 150° C.; high yield is obtained only when esters are fluorinated because of the activation for a reduction; and so forth. WO2006/106483, WO2006/106484, WO2008/065588 and Angew. Chem. Int. Ed. 2007, 46, 7473 disclose efficient ester hydrogenation reduction methods using a ruthenium catalyst including bidentate and tetradentate aminophosphine and iminophosphine as ligands. In the methods, it is necessary to use an alkali metal alkoxide as a base when the reaction is conducted. Thus, the methods have a problem that the decomposition of the compound or racemization occurs in reducing a substrate having a base-labile functional group or reducing an ester having asymmetric carbon. Meanwhile, J. Am. Chem. Soc. 2005, 127, 516, Japanese Patent Application Publication No. 2003-104993 and other documents describe a ruthenium complex including an aminophosphine or a diphosphine and a diamine as ligands. Such a ruthenium complex is reported to reduce a carbonyl group without adding a base. However, the ruthenium complex reduces only a ketone, and has difficulty in reducing an ester group that is present together with the ketone. Meanwhile, complexes as used in the present invention have been prepared by a multi-stage method as disclosed in, for example, Organomet. 2004, 23, 6239 and Organomet. 2007, 26, 5940. Specifically, in the method: an iminophosphine ligand is reduced in advance; a complex is prepared from the ligand and a ruthenium precursor; thereafter the complex is further reduced.